In civil aviation, very high frequency (VHF) amplitude modulated (AM) radio is typically used for radio voice communication between personnel in the cockpit and personnel in the air traffic control tower. VHF AM radio utilizes “simplex” (also called “half duplex”) mode: i.e. a particular channel is used for both sides of a two-way conversation, and only one party can talk at a time (also known as “two way radio”). Operators typically use the word “over” or the letter “K” to indicate that a transmission has ended and that the other party may transmit or rely on the context of the spoken message to perform this function. As there is no guarantee that a transmission has been received, protocol has been established so that the intended receiving station can confirm receipt. This protocol may include confirmation of the instructions. For example, if the control tower states “Gulfstream 234 cleared for takeoff” to the aircraft, the aircraft may reply by stating “234 cleared for takeoff” so the control tower has confirmation the aircraft has received the transmission. However, despite this protocol, lapses of communication may occur having disastrous results.
Simplex communication is used in aviation throughout the world. Consequently, if both parties transmit simultaneously, each party may believe that it has made a successful transmission, but in fact neither transmission may have been heard. In most situations, the unsuccessful transmission is discovered when neither party confirms receipt of the transmission. However, in some situations, particularly during periods of stress or emergency, the lack of confirmation may not be discovered.
An additional problem occurs when several parties are communicating as a group. With aviation radio, all pilots on a channel monitor and follow the communications of all other pilots as well as ground controllers to gain a complete picture of the situation. If a dangerous situation develops, it can be rectified as illustrated by the following example.                (Control tower): “Citation 456 cleared for takeoff, runway 30”        (Aircraft about to land): “Tower, Baron 567 is on short final for 30”        (Control tower): “Correction, Citation 456 hold short of runway, acknowledge”        (Aircraft about to take off): “456, roger, holding short, have Baron on final in sight”Unfortunately, this situational awareness is sometimes thwarted. Civil aviation voice communication uses amplitude modulation (AM) so if two stations transmit simultaneously, not only will they not hear each other, but other stations monitoring the channel will also be unable to hear either transmission. Instead, other stations will hear a squeal that represents a heterodyne between the two transmitters. Pilots are taught to avoid this situation by listening before transmitting, thus avoiding knowingly interfering with another station. However stations sometimes initiate transmission almost simultaneously and fail to realize that another station is also transmitting. In addition, operators occasionally simply make an error and transmit in the midst of another station's transmission. This is commonly described as having been “blocked” or “stepped-on.”        
One of the worst aviation accidents in history was a direct result of a “blocked” situation. In this accident, two 747 airlines collided on the runway at Los Rodeos airport in the Canary Islands killing 583 people. In civil aviation, two-way radio communication serves as an umbrella that is intended to be a final protection, however, in this case that did not happen.
In this accident, a Koninklijke Luchtvaart Maatschappij (KLM) 747 was at the end of an active runway awaiting takeoff clearance. At the same time, a Pan American 747 was taxiing down that same runway, also headed toward the takeoff position. KLM received its clearance instructions from the control tower over the radio channel, and then KLM transmitted, “We are now at takeoff.” This is not standard aviation phraseology, so the air traffic controller in the tower responded, “OK, standby for takeoff. I will call you.” The controller did not intend for KLM to begin its takeoff with Pan American still taxiing down the runway. At the same time, the Pan American crew had the same concern, so they transmitted, “And we're still taxiing down the runway . . . .” Had the controller's transmission and Pan American's transmission not overlapped, this accident would have been prevented.
Recordings from the KLM cockpit voice recorder indicated that the KLM crew heard “OK” followed by a squeal. The KLM pilot misunderstood this as a takeoff clearance and proceeded to attempt takeoff, ultimately colliding with Pan American. If the Pan American crew had realized that the tower controller had been transmitting simultaneously, they could have repeated their transmission, averting disaster. Furthermore, if the tower controller had realized that Pan American had been transmitting simultaneously, he could have repeated his transmission, averting disaster.
In response to this problem, others have proposed anti-blocking devices, typified by U.S. Pat. No. 4,932,071 (Arndt and Rutty). This device basically mechanizes the “listen before you talk” procedure, and provides some additional features. However, it has the critical limitation that once a station has commenced transmitting, it becomes deaf to other transmissions on the channel. Suppose, for example, that the controller at Los Rodeos Airport had been equipped with this device but Pan American had not. The controller could have made his transmission, exactly as he did, and still not have realized that it was blocked by Pan American transmitting a fraction of a second later. U.S. Pat. No. 4,551,854, by the same inventors, performs a generally equivalent function.
U.S. Pat. No. 4,199,661 (White and Blumke) used staggered timers to control when a particular station can seize control of the channel. This can be helpful but still give no benefit in the case where a station is blocked nonetheless. Also, in order for this system to function, all stations must be equipped with the required equipment and it must be working properly.
U.S. Pat. No. 5,566,359 (Corrigan) in effect delays the start of a transmission for a short time, first checking to see that the channel remains clear. Again, this is of no benefit if a blocking signal commences simultaneously or at a later time.
In U.S. Pat. No. 6,308,052 (Jamali and Khatib) two separate channels are used for communication, the “voice” and “service” channels. Whenever a station is transmitting on a voice channel, that station simultaneous transmits a pulsed “beacon” signal on a defined paired service channel. Because the pulsed beacon signal is random in timing and of short duty cycle, according to Jamali et al, several stations can occupy the service channel without mutual interference. Each station monitors the radio frequency (RF) power level in the service channel. If the power in the service channel exceeds some threshold, then both the service channel and the paired voice channel are assumed to be in use.
The technique disclosed by Jamali has several limitations: Firstly, the bandwidth requirement is increased; in their preferred embodiment, it is doubled. Secondly, in order to be useful, all aircraft must be equipped with the described device: no protection is obtained against aircraft not equipped with a beacon transmitter. Thirdly, Jamali et al. fails to consider the effects of not only spurious emissions from the voice transmitter, but also leakage of signals from the beacon transmitter into the beacon detector circuit. Furthermore, the transmitted power level of the beacon would be very weak, due to its low duty cycle. Due to these factors, the ability of this design to reliably detect an occupied channel would be quite limited.
A product called “Contran” is presently manufactured and sold by British Aerospace. This appears to be described in U.S. Pat. Nos. 4,549,309 and 5,566,359. It is substantially equivalent to that described by Arndt et al., described above. Its primary benefit is to prevent a transmission from starting when another transmitter is already operating.
Consequently, a device, system and/or method are needed for preventing lapses of communication in radio voice communications. Radio Technical Commission for Aeronautics (RTCA), Inc. has published document RTCA/DO-209, entitled “Minimum Operational Performance Standards for Devices that Prevent Blocked Channels Used in Two-Way Radio Communication Due to Simultaneous Transmissions”. The United States Federal Aviation Administration has adopted this standard as part of Technical Standard Order TSO-C122. The present invention meets and exceeds these requirements.